This invention in general relates to amplifier systems used to amplify one signal or a plurality of signals for transmission by active phase array antenna, and to beam-forming networks for forming such a plurality of signals, and in particular to equal power amplifier systems and beam-forming networks in the transmit antenna system of a communications satellite.
Antenna beams formed by an active phase array often require unequal excitation coefficients for the array elements. This is done to achieve either a prescribed side lobe level or to form a prescribed beam shape. Frequency addressable antenna beams transmitted by an active phase array antenna, are required for reasonably efficient operation to have in the scanning direction a narrow beam width, maximum gain, and low side lobe levels. To achieve low side lobe levels, the amplitude distribution of the signal set applied to the transmit array may ideally resemble a Taylor distribution, which is a symmetrical and tapered distribution of the type described in T. Taylor "Design of Line-Source Antennas for Narrow Beamwidth and Low Side Lobes", IRE Trans.Antennas & Propagation, pp. 16-28 (January 1955). A common method employed to achieve amplitude tapering is to amplify a common signal prior to delivery to the array with a variety of different power amplifiers. Although each of the amplifiers is operated at saturation, and thus at maximum efficiency, the system is very expensive because of the many different sizes and designs are amplifiers that are required.
Another approach is to use amplifiers of the same size or power rating, and use attenuators before the amplifiers to tailor the output of many of the amplifiers to obtain the desired amplitude distribution. Although this type of design is economical in the sense that only one type and size of amplifier is needed, the system consumes more power than it actually needs since many of the amplifiers are operated inefficiently, i.e., at below saturation. Moreover, power is typically the most precious resource on the satellite, and conservation of power often drives the entire spacecraft design. Hence, to accommodate a system using only one type and size of amplifier, the power subsystem would have to be unnaturally large to support such inefficiencies.
In contrast, other systems have employed one large amplifier and a divider network to achieve the proper amplitude tapering. The divider network consists of a series of couplers typically arranged in a corporate feed structure which divide up power from the amplifier as required to obtain the desired distribution. However, the primary disadvantages with this system is that there is only one amplifier, and the satellite is thus critically dependent on its reliability. In addition, a one amplifier system is only capable of providing a certain amount of power, and the system incurs substantial power loss in the divider network, thus limiting effective beam power.
The object of the present invention is to overcome the foregoing deficiencies. Other objects of the present invention include: providing a power amplifier system for a transmit antenna system which utilizes a plurality of identical power amplifiers operated at or near peak efficiency to form a set of amplified output signals having a desired nonlinear amplitude distribution for driving an active phase array antenna; and providing a transmit subsystem using a compact beam-forming network having a plurality of levels and adapted to provide a set of such signals to a power amplifying system having a plurality of equally sized amplifiers in a neat arrangement which minimizes crossing of transmission lines.